Fix JS source maps for optional arguments

Also, when function has expression body, use expression instead
of function declaration to mark return statement.

js/js.translator/testData/lineNumbers/complexExpressionAsDefaultArgument.kt

@@ -0,0 +1,14 @@
+fun foo(
+        a: Int =
+            when (baz()) {
+                1 -> bar();
+                else -> 0
+            }
+): Int =
+        a + 1
+
+fun baz() = 1
+
+fun bar() = 2
+
+// LINES: 2 3 4 4 5 2 8 10 12